SONAR-15424 Compute Engine fails with NPE when analysing PR without main branch

sonar-scanner-engine/src/main/java/org/sonar/scanner/scan/branch/ProjectBranches.java

@@ -22,6 +22,7 @@ package org.sonar.scanner.scan.branch;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import javax.annotation.CheckForNull;
 import javax.annotation.concurrent.Immutable;
 
@@ -30,6 +31,9 @@ import javax.annotation.concurrent.Immutable;
  */
 @Immutable
 public class ProjectBranches {
+  // matches server-side default when creating a project. See BranchDto#DEFAULT_MAIN_BRANCH_NAME
+  private static final String DEFAULT_MAIN_BRANCH_NAME = "master";
+
   private final Map<String, BranchInfo> branches;
   private final String defaultBranchName;
 
@@ -43,7 +47,8 @@ public class ProjectBranches {
         mainBranchName = branchName;
       }
     }
-    this.defaultBranchName = mainBranchName;
+
+    this.defaultBranchName = Objects.requireNonNullElse(mainBranchName, DEFAULT_MAIN_BRANCH_NAME);
   }
 
   @CheckForNull

sonar-scanner-engine/src/test/java/org/sonar/scanner/scan/branch/ProjectBranchesTest.java

@@ -70,5 +70,6 @@ public class ProjectBranchesTest {
   public void isEmpty() {
     assertThat(underTest.isEmpty()).isFalse();
     assertThat(new ProjectBranches(Collections.emptyList()).isEmpty()).isTrue();
+    assertThat(new ProjectBranches(Collections.emptyList()).defaultBranchName()).isEqualTo("master");
   }
 }